Mat;
You don't say what other account name you are logging on to.
You do sound like it's you home computer, but if not, and it's a company
computer you will have to check with your administrator and have him set it up, (If it is company very unlikely.
If it is your home computer the name your trying to sign on to must be given administror rights, this is normaly done when first setting up the computer.
You will need to go to control panel and user's, thats from xp Vista no idea,
but it should be similar.

If someone is familier with Vist they may be able to help you further, if not go to Windows help in Vista and look up administrator.

Hope this helps, let us know.
larry